Boil potatoes: place the sweet potatoes in a large pot. Add enough water to the pot to cover the potatoes half way (a little more than half way). You will need at least 3-4 inches of water but the water will not completely cover the potatoes!
Bring the water to a boil and then turn the heat to a medium simmer. Simmer them until they are fork tender. It may take 20-40 minutes to cook your sweet potatoes depending on the size of the potatoes, but you don't want them to be mushy!
Drain water and place sweet potatoes on a plate or cutting board to cool down. Once they are warm enough to hold, peel the skin from the potatoes and place them in a large mixing bowl.
Add eggs, evaporated milk, vanilla, sugar, salt and butter and mash everything together until smooth. I start with a potato masher, and then use an electric mixer on low speed, for about 45 seconds, to get the mixture really smooth.
Pour mixture into a lightly greased casserole dish. Anything around the size of an 8x11'' , 9x13'' or even a deep dish 10'' pie pan would all work great!
For the topping:
Add flour, brown sugar and pecans to a mixing bowl.
Add butter and cut in with a fork or pastry blender until well combined.
Sprinkle topping over sweet potato mixture.
Bake at 350 degrees F for 35-40 minutes.
*Topping: To add a marshmallow topping instead, simply top the casserole with a layer of miniature marshmallows and bake for about 25 minutes, or until the marshmallows are puffy and browned. To Make-ahead: You can prepare sweet potato casserole 1-2 days in advance. First make the filling and smooth into your baking dish. Then make the topping but instead of putting it on the casserole, place it in a ziplock bag. When you're ready to bake your sweet potato casserole, take it out of the fridge and sprinkle the topping on top! Then bake at 350 for 35-40 minutes.This is the perfect Thanksgiving side dish to make a day or two ahead of time, to save time and stress the day of. Here are some other great make-ahead Thanksgiving recipes.Storing: Store sweet potato casserole covered in the refrigerator for 4-5 days.Freezing: Prepare the sweet potato casserole completely, but don't bake it. Cover with a double layer of aluminum foil and freeze for up to 3 months. Thaw overnight in the refrigerator. Allow it to come to room temperature for 30 minutes on the counter before baking.